Some of the most precise measurements of physical quantities currently available derive from atom interferometry. These include limits on composition-dependent gravitational forces (Einstein's principle of equivalence), measurements of the ratio h/m in an atom, and derivative determinations of the fine structure constant. Measurements of such exquisite precision, as well as accelerometer and gyroscopic applications, require stability in the face of external factors such as temperature or pressure variations and other mechanical or optical perturbations.
Atom interferometry encompasses various classes of well-known techniques, and has been employed not only for fundamental measurements but for such practical applications as precision inertial sensing. Atom interferometry is based on tracking the center-of-mass motion of an ensemble of atoms by generating matter wave interference and measuring its phase shift. The matter wave interference may be created by applying light pulses that interact with the atoms via two-photon Raman transitions. Critical components of an atom interferometer include a source of atoms (thermal or cooled) in a vacuum chamber, a defined trajectory, and counter-propagating Raman beams that, along with the trajectory, define the inertially sensitive axes. As the term is used herein, an axis shall be designated “inertially sensitive” if motion about or along the axis by the atom interferometer results in a detectable interferometer signal.
Depending on its configuration, an atom interferometer may be operated as an accelerometer, a gyroscope, or a combined accelerometer-gyroscope. In either of the latter two gyroscopic cases, a baseline set by launching the atoms at a finite velocity (typically on the order of m/s to hundreds of m/s) is necessary to provide rotational sensitivity. (Various publications treat the sensitivity-limiting parameters of an atom interferometer. Examples include Canuel et al., “Six-axis inertial sensor using cold-atom interferometry,” Phys. Rev. Lett., vol. 97, 010402 (2006)) where sensitivity was considered limited by the cold atom sources, and Sorrentino et al., “Sensitivity limits of a Raman atom interferometer as a gravity gradiometer,” Physical review A, vol. 89 (2014), and Yver-Leduc et al. “Reaching the quantum noise limit in a high-sensitivity cold-atom inertial sensor,” Journal of Optics B: Quantum and Semiclassical Optics, vol. 5, p. 5136 (2003), all of which are incorporated herein by reference. A cold atom interferometer for navigation applications was also recently addressed by Battelier et al., “Development of compact cold-atom sensors for inertial navigation,” arXiv preprint arXiv:1605.02454 (2016) (hereinafter, Battelier 2016), incorporated herein by reference.
Since both accelerometer and gyroscope sensitivities scale quadratically with the time between interactions with the Raman pulses, there is a fundamental trade-off between sensitivity and sensor volume and bandwidth. In any practical configuration, external factors bearing on the relative optical phase of successive laser beams impinging on a probed ensemble of atoms give rise to measurement perturbations and drift, ultimately limiting system sensitivity. Thus, a configuration that, by its nature, provides for long-term stability of the relative phase of counterpropagating probe beams is particularly valuable. Such a configuration is taught for the first time herein.
Magneto-optical traps (MOTs) are widely used as sources of cold, dense clouds of atoms, and, recently, of simple molecules as well. Double traps are used to generate cold atomic ensembles swapped between the traps and interrogated interferometrically during transit between the traps. Such a system is described, for example, by Rakholia et al., “Dual-axis high data-rate atom interferometer via cold ensemble exchange,” Phys. Rev. Appl., vol. 2, 054012 (2014) (hereinafter, Rakholia 2014), which is incorporated herein by reference.
For practical applications in inertial navigation systems (such as gimbaled and, especially, strapdown platforms), it is desirable for the atom interferometer to have a long-term stability that exceeds the performance of state-of-the-art classical sensors. Additionally, the instrument needs to be portable, orientation-insensitive, and be operated to meet its sensitivity and short-term noise requirements, while minimizing the “dead time” associated with generating the atom source (e.g. loading atoms into magneto-optical traps) and the sampling time (set by time between Raman pulses).
Mechanically rigid geometries that have been employed in single-trap designs do not lend themselves to double-trap implementation. The design of single MOTs employing micromirrors etched into monolithic structures, for example, has been driven largely by the goal of miniaturization and achieving a trap on a chip. Micromirrors etched into a pyramid, for example, have been used to achieve a tetrahedral four beam MOT, as described by Vangeleyn et al., “Single-laser, one-beam, tetrahedral magneto-optical trap,” Opt. Exp., vol. 17, pp. 13601-08 (2009), which is incorporated herein by reference.
The proximity of two MOTs in such configurations does not allow for implementation of integral micromirror geometries as discussed above with respect to single traps. Typically, the proximity of the two MOTs, dictated by achievable magnetic field gradients, has been on the order of 20-50 mm. Therefore, dual MOT sensors have exclusively employed discrete beam-splitting and beam-turning optics deployed at a substantial distance from the traps. Performance of sensors based on two-trap MOTs may be limited by the stability of the respective traps. Consequently, a novel physical mechanism for ensuring the anti-symmetry of the MOTs, differing only in the sign of the wavevector of the launched atoms, while not limited by the structural rigidity of a supporting base, is highly desirable.
Some steps addressing various of these limitations have already been suggested in the prior art. First, the dead time in the atomic measurement associated with generating the atom source can be reduced by running two reciprocal cold atom interferometers in a “launch-catch” configuration and recapturing the atoms (at rates up to ˜100 Hz) in between measurement cycles, as in Rakholia 2014. In that configuration, inertial sensitivity can be enhanced by applying Raman pules using physically separated Raman beams which extend the gyroscope baseline. However, that increases the complexity of distributing the Raman beams and maintaining phase stability.
The overall dead time in the inertial measurement can be effectively eliminated by operating the atomic sensor in closed loop with classical sensors that are co-mounted on the same moving platform (as in Battelier 2016, for example). Finally, orientation-insensitivity under acceleration can be reduced by using counter-propagating Raman beams with identical effective k-vectors oriented perpendicular to the axis of launch. This requires a Raman geometry in which the two Raman frequency components are incident from opposite sides of the cell containing the ensemble of atoms rather than a geometry in which both components together pass through the cell and retro-reflect back through it.
Propagation of beams of distinct wavelengths in orthogonal propagation modes of polarization-maintaining optical waveguide (such as optical fiber) has been the basis of various devices as described, for example, by Feng et al., “Single-Polarization, Switchable Dual-Wavelength Erbium-Doped Fiber Laser with Two Polarization-Maintaining Fiber Bragg Gratings,” Optics Express, vol. 16, pp. 11830-11835 (2008), which is incorporated herein by reference. Additionally, papers such as Ménoret et al., “A transportable cold atom inertial sensor for space applications,” International Conference on Space Optics, October 2010, Rhodes, Greece. pp. 1-4, (2010), and Ménoret et al., “Dual-Wavelength Laser Source for Onboard Atom Interferometry,” Opt. Lett., vol. 36, pp. 4128-4130 (2011), both incorporated herein by reference, teach the use of separate wavelengths generated within a fiber laser. However, two-trap atom interferometers have heretofore required that Raman beams of distinct wavelength be delivered separately because of the complexity entailed in combining and separating disparate wavelengths to multiple traps.